Francisella tularensis is the causative organism of the disease tularemia. The extreme pathogenic and infectious properties of this organism, as well as its ease of dissemination, make it a plausible candidate for use in a biological weapon. F. tularensis can be rendered even more dangerous by its aerosolization and by transformation with plasmids encoding resistance to antibiotics. It is therefore imperative to explore novel therapies for inhaled F. tularensis that do not rely on the use of these drugs. Developing such therapies would be facilitated by an improved understanding of the host genes participating in the clearance of F. tularensis from the lungs of experimental mice. Toward that end, I propose to use two parallel gene identification strategies: quantitative trait locus (QTL) mapping, and microarray-based gene expression studies. Individually, each of these approaches yields more candidate genes than can be feasibly pursued within the time limits of this proposal. To overcome this problem, I will reduce the candidate gene number to a manageable level by selecting only those genes that are contained within identified QTL and that are induced by F. tularensis infection. This comparatively small subset is likely to contain genes causally associated with susceptibility to F. tularensis. These genes will be tested for their biologic relevance by selectively suppressing them in cultured macrophages using short interference RNA (siRNA). Macrophages transfected with gene-specific, siRNA-expressing plasmids will be compared to their untransfected counterparts for their abilities to support proliferation of F. tularensis in vitro. Biologically validated genes emerging from these studies will represent novel molecular targets whose actions might be manipulated by therapies designed to improve the innate immune response to F. tularensis.